The switch may indeed be the issue. Any chance you’re using Cisco managed switches (SG200 / SG300)?
This all seems very familiar to me as I beat my head against the wall with a UPnP server running on a QNAP that was simply invisible on the network. Turns out that the QNAP and the switch were getting into a fight regarding multicast packets and the switch was winning by simply dropping them.
Roon/RAAT uses multicast to find audio endpoints and I wonder if these packets are getting dropped in a similar way in your setup.
If you are using a managed switch then the easiest way to test would be to substitute an unmanaged switch and see if it works. Otherwise check the configurations of the (network) devices on your network (including those which aren’t seemingly in play here) and disable any reference to IGMP snooping / multicast routing / UPnP.
It’s also worth checking your QNAP to ensure that jumbo frames are disabled. This shouldn’t make a difference for device discovery, but has been known to cause some strangeness in similar configurations.